Printed-circuit feed for reflector antennas

ABSTRACT

A printed-circuit feed employs a pair of slot radiators disposed on opposite sides of an axial point at which the feed is coupled to an axially extending coaxial line. The slots are formed in a conductive coating on the forward side of a dielectric-sheet base having a feed-conductor pattern printed on the rearward side, which is backed by a foam-filled cavity. The slots are fed in opposite phase to produce in-phase electric field polarization of the radiation from the respective slots.

United States Patent [1 1 i [1 11 337mm Phillips Nov. 6, 1973 [54] PRINTED-CIRCUIT FEED FOR REFLECTOR 3,691,563 9/1972 Shelton 333/84 M ANTENNAS [75] Inventor: James P. Phillips, Orland Park, 111. P i Examiner Eli Lieberman [73] Assignee: Andrew Corporation, Orland Park, Freed-mick Leydig gt 22 Filed: Sept.ll, 1972 211 Appl.N0.:287,983

[5 7 ABSTRACT A printed-circuit feed employs a pair of slot radiators disposed on opposite sides of an axial point at which [52] US. Cl, 343/770, 343/840, 343/872,

the feed is coupled to an axially extending coaxial line.

. 333/84 The slots are formed in a conductive coating on the for- H01q 13/10 ward Side of a dielectric sheet base having a feed [58] Field of Search 343/770, 771, 781,

conductor pattern printed on the rearward side, which 343/840 333/84 M is backed by a foam-filled cavity. The slots are fed in opposite phase to produce in-phase electric field polar- [56] References Cited ization of the radiation from the respective slots.

UNITED STATES PATENTS 2,505,424 4/1950 Moseley 343/781 7 Claims, 8 Drawing Figures PMENTED HUV 61973 SHEET 10F 2 Y 1 PRINTED-CIRCUIT FEED FOR REFLECTOR ANTENNAS This invention relates to feeds for reflector antennas, and more particularly to printed-circuit feeds.

One of the simplest forms of reflector antenna feed now in widespread use is the coaxial horn feed, wherein a short cup-shaped waveguide length or open-ended cavity is fed by an internal dipole excited by a coaxial line which extends from the center of the reflector dish. The feed is supported by a rigid tubular boom, either containing a flexible coaxial cable or itself constituting the outer conductor of a rigid coaxial line. Such a construction is substantially less expensive than waveguide-fed horns, in addition to minimizing aperture blockage by the transmission line employed for coupling to the feed. As in the usual use of a circular horn, the polarization of the radiation is substantially the TE mode of circular waveguide.

It is the object of the present invention to provide a feed construction having mode and pattern characteristics, and simplicity of mechanical mounting and electrical coupling, similar to those of the coaxial horn, but of still lower cost, particularly in commercial production quantities. One known manner of simplifying the production of various forms of conductive radiating feed structures is the employment of printed-circuit techniques. Printed conductors have heretofore been employed as substitutes for conventionally fabricated conductors in certain forms of feed earlier constructed with wires, rods, and the like, as well as in specialpurpose feeds employing slot radiators. However no such feed is known which satisfactorily produces the TE polarization and the dish-illumination pattern of the coaxial horn feed while at the same time being capable of being mounted andelectrically coupled by a single, rigid axial support and transmission line.

The construction of the present invention employs a radiating structure having slots 'on opposite sides of the center of a planar conductor which constitutes the front wall of a cavity, the latter being filled with a foam electric to eliminate any necessity of provision of gas seals. The planar conductor in which the slots are formed is connected to the outer conductor of a coaxial transmission line like that'used with the coaxial horn.

The inner conductor of the coaxial'line is coupled within the cavity to the oppositely located slots, with means being provided to feed the slots in phase opposition, i.e., with 180 more delay in the coupling from the coaxial line inner conductor to one slot than to the other slot. With the opposed orientation of the slots with respect to the common center feed-point, this results in in-phase radiation from the slots at any remote point.

The slotted planar conductor is formed as a printedcircuit pattern on the outer or forward face of a circuit board backed by the foam-filled cavity, and a feedconductor pattern is printed on the inward or rearward face of the board to form an asymmetric strip-line feed network for the slot radiators. The desired relative phase delay is produced by elongating the strip-line feed for one of the slots with respct to that of the other. If so desired, conductive connections may be made through the board in coupling to the slots on the outer or forward surface. However it is more advantageous to line feed is enlarged as it crosses the slot radiator to increase the frequency band of operation. Often to match the strip-line impedance to the slot radiating impedance requires increasing the width of the stripline conductor at the input to the slot coupling to form a parallel plate capacitor.

The above'brief description of the construction, together with further structural features and advantages provided by the invention, will be best understood in connection with description of the embodiment of the invention illustrated in the drawing, in which:

FIG. 1 is a side view in elevation, partially broken away in section, showing a feed embodying the invention mounted on a fragmentarily illustrated reflector dish;

FIG. 2 is an exploded view of the feed of FIG. 1;

FIG. 3 is a view in front elevation of a circuit board constituting a portion of the feed, showing a slottedconductor pattern printed on the forward surface;

FIG. 4 is a view in rear elevation of the circuit board, showing a strip-line conductor pattern printed on the rear surface;

FIG. 5 is a schematic equivalent circuit of the stripline pattern of FIG. 4;

FIG. 6 is a schematic illustration of the electric field pattern or mode produced by the feed;

FIG. 7 is an enlarged detailed view showing the manner of connection of a coaxial cable employed in the illustrated embodiment to the conductor patterns of the printed circuit board; and

FIG. 8 is a diagrammatic showing a fundamental or basic aspect of the construction.

A feed assembly in accordance with the invention is shown in FIG. I mounted from the center of a fragmentarily illustrated parabolic reflector dish 10. A mounting flange l2 (fastened in-a conventional manner not illustrated) is employed for positioning the feed assembly with a rigid tubular support boom 14 extending through a central aperture 15 in the dish. A coaxial cable 16 extends through the boom and terminates in a suitable connector '18 behind the reflector. At the end of the boom 14 at the focus of the reflector is disposed a feed generally designated 20, comprising -a cupshaped metal cavity 22 filled with a foam dielectric filling 24, the open forward'erid or mouth being closed by a circular printed-circular board 26, the front or outer face of which is covered by a conically tapered foam protective cover or radome 28 encircling the end of the boom 14.

The printed-conductor patterns on the forward or external and the rearward or internal surfaces of the board 26 are shown in FIGS. 3 and 4, respectively. As seen in these FIGS. and in FIG. 2, the board has diametrically opposed apertures 30 therethrough. These apertures are'provided for convenience of fabrication,

the foaming material comprising the filling of the cavity being introduced through them after the board 26 is assembled with the cavity cup 22; as hereinafter seen, these apertures are in a region wherein they have very small electrical effect, so that they may be disregarded for purposes of understanding the construction effective in forming the radiation pattern.

The front or outer surface of the board has an allover conductive coating 32 exceptfor a small open center portion 34 and relatively large radiation slots 36 and 38 symmetrically disposed on opposite sides of the center, the insulating core or base 40 of the circuit board of course being exposed by this slotting pattern of the coating. The slots 36 and 38 are substantially identical, each extending transversely of a diameter about which it has mirror-image symmetry, while mirror-image symmetry is also presented along all other diameters. A small aperture 42 extends through the insulating base 40 at the center. As best seen in FIG. 7, the outer conductor 44 of the coaxial cable 16 is conductively bonded by solder to the front conductive coating 32 of the board, and the inner conductor 46 extends through the aperture 42-and is conductively bonded by solder to the center of the printed conductor pattern 48 on the rear or inner surface of the board within the cavity, this pattern serving as a strip-line feed circuit for the slots 36 and 38. The tip of the inner conductor 46 is hollowed to facilitate soldering with a minimum of melting of the dielectric of the cable.

The components or portions of the printed strip-line slot coupling network shown in FIG. 4 will best be understood by simultaneous consideration of the schematic equivalent circuit of FIG. 5. Understanding of these will be further facilitated by first considering the more elementary or basic form of the invention shown schematically in FIG. 8. In this Figure, slots 36a and 38a are similar (except in shaping later discussed) to the slots 36 and 38. Conductors 50 and 52 are eimployed to couple the end of the coaxial cable inner conductor 46 to the slots. Were the coupling to the respective slots identical and symmetrical, the radiations from the respective slots would be of the same phase as viewed from the common central feed-point but of course of opposite phase at appreciable distances. In the construction of FIG. 8, the strip-line conductor 52 is one-half wavelength longer at the operating frequency than the conductor 50, this being accomplished by a folded or tortuous configuration of the former as opposed to the short straight extension of the latter. Thus as viewed from the center, the slot excitations are in opposite phase, the inner edge of one slot being in phase with the outer edge of the other, while at remote points the slot excitations are in phase. The remote inphase relation may of course be accomplished in other manners, as by using the same delay or line-length in the feeding of both slots, but feeding oneslot at the inside edge and the other slot at the outside edge, or by employing a phase-shift device other than difference in strip-line length. However the employment of the linedifferential delay is superior to such alternatives when both simplicity of fabrication and ease of obtaining full symmetry of the radiated pattern are considered.

The simple center-feeding of the slots shown in FIG. 8 produces difficulties in matching the slot impedance to those of the strip-line feed and the coaxial cable, because of the high impedance at the center-point of the slot. Off-center coupling to the slot at a single point of desirably matched impedance produces difficulty in making the radiation pattern wholly symmetrical because of the relatively small size of the ground plane in which the slots are formed. The employment of a lowimpedance center-coupling such as a T-bar coupling extending longitudinally of the slot may be utilized, but parallel and symmetrical feeding of each slot at both end regions, the parallel impednace matching the line, is generally found capable of greater bandwidth.

The parallel or dual end-feed construction just described is shown in the embodiment of FIG. 4 and the equivalent circuit of FIG. 5. Extending generally lateremployed with constructions wherein ally outward from the center aperture 42 are feed conductors 54 and 56, the ends of which are directly coupled to capacitance plates or enlargements 58 and 60 opposite the end portions of slot 36 and are coupled to similar capacitance plates or enlargements 62 and 64 opposite the end portions of slot 38 through extension or path-lengthening conductors 68 and 70, the latter each providing a half-wave relative delay. Extending from each of the capacity enlargements 58, 60, 62 and 64 is a bandwidth-enlarged slot-coupling 59, 61, 63, and 65, terminating in a respective quarter-wave stripline conductor extension 72, 74, 76, and 78. These open-circuited quarter-wave strip-lines act as effective shorts at their ends, so that the operation is the same as though the ends of the coupling portions 59, 61, etc., were conductively connected through the insulating board to the ground plane at the outer edges of the slots.

To produce simulation of the TE round waveguide mode, the inner edges of the slots 36 and 38 are curved, the curvature being somewhat less than an arc centered on the center of the board. The shape of the composite electric field pattern radiated by the pair of slots is thus closely simulative of the TE electric field pattern as shown by dotted arrows in FIG. 6. As in the case of the coaxial feed, the termination of the electric field vectors generally radially of the support boom 14 minimizes the effect of the presence of the boom on the pattern which illuminates the reflector. Optimum shaping of the slots for this purpose is of course done by experimentation. In the slot-shape shown in the drawing, the outer edges of the slots are linear, but these likewise may advantageously be curved if so desired, the optimum shaping of the inner edges being slightly altered from that illustrated in this case.

A variety of dimensional and construction details may be employed utilizing the invention, particularly in its broader aspects. The particular configuration illustrated in the drawing employs a circuit-board of a diameter of approximately three-quarters of a wavelength (free-space) at the operating frequency. With a SO-ohm feed cable, the width of the conductors 54 and 56 is selected to give each strip-line thereby formed an impedance of 70.7 ohms and that of each of the further branches 68, 70, etc., gives a line impedance of ohms, the impedance at each point of coupling to a slot likewise being of this latter value. Similar selection of conductor widths for impedance matching should be the slotcouplings differ from the balanced dual feed-point construction here illustrated.

The invention is particularly well-adapted for use in the 1.5 to 4.0 GHz range of frequencies. Highly satisfactory radiation patterns and VSWR are obtained with diameters as small as a half-wavelength or less, and the printed-circuit fabrication makes individual tuning and the unnecessary. A commercial construction for use with a four foot reflector in the band from 2.5 to 2.7 GHZ employs a 3/32 inch thick polystyrene insulating board with conducting coatings of copper of 1 ounce per square foot thickness, with a spun copper cavity cup and polyurethane filling and radome. A support boom of 1 inch diameter is employed with a one-half inch SO-ohm corrugated coaxial cable extending therethrough.

Persons skilled in the art will readily devise substantially different-appearing feeds which nevertheless utilize the teachings of the invention. Accordingly, the scope of the patent protection to be afforded the invention should not be limited to the particular embodiments specifically disclosed, but should be determined only in accordance with the definitions of the invention in the appended claims, and equivalents thereto.

What is claimed is:

l. A feed for reflector antennas comprising a planar conductor having substantially identical radiator slots symmetrically disposed on opposite sides of the center, a conductive cavity enclosing the rearward surface of the conductor, a coaxial feed line extending forward from the center and having its outer conductor connected to the planar conductor and its inner conductor extending therethrough, and means within the cavity coupling the inner conductor of the coaxial line to the respective slots with their corresponding edges excited in opposite phase to produce in-phase electric field orientation of radiation from the slots at distant points.

2. The feed of claim 1 wherein the planar conductor is a coating on the forward side of an insulating board and the coupling means for the slots comprises conductors coated on the rearward side of the board and extending from the center to corresponding regions opposite the respective slots, the conductors coupling to the respective slots having a difference in length of onehalf wavelength to produce the phase difference.

3. The feed of claim 2 wherein the conductors have enlarged portions at least partially opposite the slots.

4. The feed of claim 2 having a pair of conductors coupled to each slot at symmetrically related regions thereof.

5. The feed of claim 2 having the outer ends of each of the conductors extending substantially a quarterwavelength beyond the corresponding slots, to form an effective short circuit through the board without conductive connection therethrough.

6. The feed of claim 2 wherein the cavity and the board are circular, the slots each having at least one curved edge to produce a field pattern simulating the TE mode of circular waveguide.

7. The feed of claim 1 having a rigid tubular support extending forward from the center and enclosing the coaxial line. 

1. A feed for reflector antennas comprising a planar conductor having substantially identical radiator slots symmetrically disposed on opposite sides of the center, a conductive cavity enclosing the rearward surface of the conductor, a coaxial feed line extending forward from the center and having its outer conductor connected to the planar conductor and its inner conductor extending therethrough, and means within the cavity coupling the inner conductor of the coaxial line to the respective slots with their corresponding edges excited in opposite phase to produce in-phase electric field orientatioN of radiation from the slots at distant points.
 2. The feed of claim 1 wherein the planar conductor is a coating on the forward side of an insulating board and the coupling means for the slots comprises conductors coated on the rearward side of the board and extending from the center to corresponding regions opposite the respective slots, the conductors coupling to the respective slots having a difference in length of one-half wavelength to produce the phase difference.
 3. The feed of claim 2 wherein the conductors have enlarged portions at least partially opposite the slots.
 4. The feed of claim 2 having a pair of conductors coupled to each slot at symmetrically related regions thereof.
 5. The feed of claim 2 having the outer ends of each of the conductors extending substantially a quarter-wavelength beyond the corresponding slots, to form an effective short circuit through the board without conductive connection therethrough.
 6. The feed of claim 2 wherein the cavity and the board are circular, the slots each having at least one curved edge to produce a field pattern simulating the TE11 mode of circular waveguide.
 7. The feed of claim 1 having a rigid tubular support extending forward from the center and enclosing the coaxial line. 